Color images are usually obtained by the reaction of the oxidation product of silver halide and an aromatic primary amine color developing agent with dye forming compounds known as couplers in conventional photographic techniques. By the reaction of the coupler and the oxidation product of a color developing agent, the oxidized color developing agent is coupled at the coupling position of the coupler to form a dye. The dyes formed by coupling are indoaniline dyes, azomethine dyes, indamine dyes, or indophenol dyes, according to the chemical compositions of the coupler and the developing agent.
In a multicolor photographic element, a subtractive color photographic process is usually employed for the formation of color images and the dyes formed by coupling are usually cyan, magenta and yellow dyes formed in silver halide emulsion layers having light sensitivity for radiation absorbed by image dyes, that is, silver halide emulsion layers having light sensitivity in the red spectral region, the green spectral region, and the blue spectral region, respectively, or layers adjacent to these emulsion layers.
For shortening the processing time for color photographic materials, color photographic materials are processed at temperatures higher than normal temperatures (20.degree. to 25.degree. C.), and at present are usually processed at temperatures higher than 30.degree. C. However, in a high temperature quick processing, a hydrophilic colloid such as, for example, gelatin in which silver halide, a color coupler, and other additives are dispersed is softened and swelled, whereby the emulsion layer is easily damaged or peeled off from the support. Accordingly, silver halide emulsion layers must be hardened to withstand high temperature processing. Hardening of emulsion layers is attained by introducing a photographic material having the emulsion layers in a hardening bath during development processing or incorporating a hardening agent such as formaldehyde, dialdehyde, mucochloric acid, etc., in silver halide emulsions during the preparation thereof.
It is important for hardening photographic layers to attain the maximum hardness as quickly as possible after drying in order to minimize the change of permeability of a developer.
Recently, it has become important to use a hardening agent which acts quickly (e.g., within about one day) for photographic purposes in place of a hardening agent which acts over a long period of time (e.g., over about one day, such as 2 weeks). By the quick speed of the hardening reaction, the change of photographic materials during the storage thereof can be avoided and also the continuous reduction of the permeability of the emulsion layers for a developer can be avoided.
The quick acting hardening agent is a compound capable of completing a cross-linked bond of gelatin in a very short period of time, if possible during coating and drying emulsion layers. The maximum cross-linked bond is attained within 24 hours.
Useful quick acting hardening agents include, for example, carbodiimides as reported in Erich Schmidt, Fritz Hitzles, Eberhard Lahde, Berichte der Deutschen Chemischen Gesellschaft, Vol. 71 II, p. 1933 (1938) and Bull. Soc. Chim. France, p. 1360 (1956); dihydroquinone compounds as described in German Patent Application (OLS) No. 2,322,317; carbamoylpyridinium compounds as described in German Patent Application (OLS) Nos. 2,225,230, 2,317,677 and 2,439,551; and carbamoyloxypyridinium compounds as described in German Patent Application (OLS) No. 2,408,814.
One of the features common to these whole quick acting hardening agents is that a carboxy group is activated. This action can be shown by a known reaction example of carbodiimide and a carboxylic acid. In the reaction, N-acylurea or an acid anhydride is used as the active group. In the case of a carboxy group and protein having an amino group, the reaction further proceeds and the activated carboxy group forms a peptide bond with the amino group. Such a compound is, therefore, known as a peptide reagent (Chemical Review, Vol. 67, pp. 107 to 152 (1967)).
However, such a type of compound reduces development cf a magenta layer, which therefore restricts the utilization of the compound as a quick acting hardening agent for color photographic materials.